The present invention relates to a vertical cavity laser and more particularly, the present invention relates to the unique combination of wafer fusion and selective oxidation to form a long wavelength vertical cavity laser.
The major obstacle for the fabrication of long-wavelength (1.3-1.55 xcexcm) vertical cavity laser, VCL, is the lack of high reflective InP-based Bragg mirrors as well as the difficulty to realize effective electrical and optical confinement schemes The technology of wafer fusion as discussed in a paper entitled xe2x80x9cDouble-Fused 1.52 xcexcm Vertical-Cavity Lasers,xe2x80x9d published in Appl. Phys. Lett, Vol. 64, (1994), p1463, is one solution to solve the mirror problem by utilizing GaAs/AlGaAs mirrors. Nearly all room temperature operating LW-VCLs (long wave vertical cavity lasers) in the art today make use of one or two wafer fused GaAs based mirrors.
Selective oxidation can be applied in a mesa structure, double fused VCSEL, realizing small current aperture in the p-side as reviewed in the article, xe2x80x9cLaterally Oxidized Long Wavelength CW Vertical Cavity Lasers,xe2x80x9d published in Appl. Phys. Lett., Vol. 69 (1996) p471. Very high operation temperatures (63xc2x0 C. (cw.) and 120xc2x0 C. (pulsed)) have been achieved with this technique as delineated in the paper, xe2x80x9c120xc2x0 C. Pulsed Operation From A 1.55 xcexcm Vertical Cavity Laser,xe2x80x9d presented at the 1997 LEOS Summer Topical Meetings, Montreal, Canada. As a restriction, the structure involves two wafer fusion steps and two wafer fused heterojunctions inside the cavity, which complicate the fabrication process and generally affect the reliability of the device.
Very good results have been demonstrated recently with a single fused 1.3 xcexcm device with one dielectric top mirror. This topic was reviewed in the article, xe2x80x9cSubmilliamp 1.3 xcexcm Vertical-Cavity Surface-Emitting Lasers With Threshold Current Density less than 500 A/cm2xe2x80x9d, Electron. Lett., Vol. 33, (1997) p1052. Current confinement here is realized by oxygen implantation into a p-doped GaAs layer on a GaAs-based DBR. Submilliamp thresholds and threshold current densities around 600 A/cm2 have been demonstrated with those devices. However, the oxygen implant sets some limits for the minimum laser diameter and might leak at temperatures above 40xc2x0 C.
The present invention overcomes the constraints of the prior art methods and in accordance with one embodiment of the present invention, there is provided a vertical cavity laser, comprising:
a gallium arsenide semiconductor substrate body having a bottom surface and a top surface;
a planar contact on the bottom surface;
a mirror stack on the top surface, the mirror stack composed of a plurality of layers of GaAs and AlGaAs;
an active layer comprising multiple quantum well structure, the structure embedded cladding layers;
a plurality of channels in an oxidized layer of the mirror stack and the active layers, the channels in optical communication with the active layers and the mirror stack;
a dielectric mirror; and
a ring contact surrounding the mirror.
The laser is formed by replacing oxygen implantation with a pre-fusion oxidation of an Al(Ga)As layer for electrical isolation. The lateral profile of the Al-oxide/semiconductor interface can be designed by changing the (low) Ga-content vertically within the Al(Ga)As oxidation layer as set forth in the article, xe2x80x9cEstimation of Scattering Losses in Dielectric Apertured Vertical Cavity Lasersxe2x80x9d, printed in App. Phys. Lett., Vol. 68 (1996), p1757 and in xe2x80x9cScattering Losses From Dielectric Apertures in Vertical-Cavity Lasersxe2x80x9d, Journal of Selected Topics in Quantum Electronicsxe2x80x9d, (1997), p379. In analogy to short wavelength VCLs with oxide-apertures, small VCL diameters and thus very low threshold currents and high efficiencies can be realized. As in short wavelength devices, electric isolation can be maintained up to high temperatures.
The combination of the two technologies as discussed above result in a number of advantages. In comparison to the double fused VCL, the proposed structure offers a reduction in processing complexity by omitting the second fusion step. This improves the reliability of the device due to the reduced number of wafer fused heterojunctions inside the laser resonator. The epitaxial structure is planar and top emitting which is desirable for testing and packaging. The ring contact is placed on the n-side of the device where it benefits from the high mobility of the electrons. This assures a homogeneous current injection through the p-side oxygen current aperture and releases the demands on the otherwise critical top mirror dimensions. Furthermore, the Al-oxide is embedded inside the structure and thus automatically xe2x80x98sealedxe2x80x99.